This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Usually, there are two kinds of scheduling schemes (also referred to as HARQ index allocation), dynamical scheduling and Semi-Persistent Scheduling (SPS).
A HARQ feedback (ACK/NACK) position (e.g., indicated by a Physical Uplink Control CHannel (PUCCH) HARQ index) for dynamical scheduling is derived according to Control Channel Element (CCE) index. For example, providing one to one mapping relationship between PUCCH HARQ indices and CCEs, a PUCCH HARQ index may be calculated based on a CCE start index of Downlink Control Information (DCI) in Physical Downlink Control CHannel (PDCCH), i.e., PUCCH HARQ Index=mapping_func (CCE start index).
For SPS, due to no DCI indication, HARQ feedback position of DownLink (DL) periodical data can't be derived from its CCE index like the dynamic scheduling does. So, the 3GPP specifies that a BS needs to statically allocate up to 4 candidate positions in a Radio Resource Control (RRC) message, from which a specific one is chosen and indicated to the UE via 2 bits Transmit Power Control (TPC) field in DL DCI at SPS (re)activation. That is, SPS has statically allocated HARQ index for HARQ feedback position of DL periodical data.
Due to coexistence of the different HARQ index allocation methods, there is high probability of HARQ index confliction between dynamic scheduling and SPS. For example, the dynamically scheduled UE's HARQ index derived from its CCE index happens to hit the statically allocated one for SPS UE, especially under heavy load. In such a case, either the dynamically scheduled UE is postponed to next System Frame (SF) or SPS UE moves to another HARQ index within the at most 4 candidates. However, regardless of which option is chosen, the flexibility will be impacted and extra overhead (e.g., Downlink Control Information (DCI) signaling) is added.
Although there exist some workarounds to avoid such potential confliction between dynamic scheduling and SPS, such workarounds all introduce side effect. For example, allocation of one more area in PUCCH format 1 dedicated for SPS HARQ feedback without overlapping with that for dynamic part, however, need consume extra PUCCH resources.
Moreover, there is one-to-one mapping between CCEs and PUCCH HARQ indices. Considering the number of scheduled UEs per Transmission Time Interval (TTI) is much smaller than the maximum number of CCEs, the HARQ index allocation is actually very sparse in PUCCH format 1a/1b area. This results in low PUCCH resource usage efficiency. In a future evolution version such as the 5th Generation (5G), the inefficiency may become worse due to the wider frequency spectrum (e.g., 100 M in 5G), shorter TTI (e.g., 200 us in 5G) as well as larger capacity.